1. Field of the Invention
The present invention relates to an exhaust gas purification device of an internal combustion engine.
2. Description of Related Art
In some cases, an occlusion reduction type NOx catalyst (a NOx catalyst, a Lean NOx Trap, LNT) is arranged in an exhaust pipe of a diesel engine or the like in order to purify nitrogen oxides (NOx) in exhaust gas. The NOx is occluded to the LNT in a lean atmosphere, which is a basic atmosphere in the diesel engine. If the atmosphere is switched to a rich atmosphere after an interval, the occluded NOx is reduced into the harmless nitrogen and is discharged.
There are known methods for forming the rich atmosphere such as rich combustion for forming a rich gas in an engine cylinder, post-injection for injecting fuel into the cylinder after the combustion is almost completed, and exhaust gas fuel addition for injecting unburned fuel as a reducing agent directly to the LNT by a fuel addition valve provided to the exhaust pipe.
There are various kinds of proposals about improvement of exhaust gas purification performance of the LNT. For example, following Patent document 1 (JP-A-2006-336518) describes that a system equipped with a NOx catalyst determines a degradation state of the NOx catalyst from a sensing value of an oxygen concentration sensor and sets length of a lean period.
As compared to the post-injection or the exhaust gas fuel addition, the rich combustion has following advantages, for example. That is, the rich combustion can supply the reducing agent having relatively high reducing efficiency and requires only small quantity of the added fuel as compared to the post-injection or the exhaust gas fuel addition. However, the rich combustion has a problem of a torque step (torque shock). The problem of the torque step is a problem of difference in the torque value between the lean combustion period and the rich combustion period.
Conventionally, in many cases, the diesel engine is equipped with an EGR pipe for recirculating the exhaust gas from the exhaust pipe to the intake pipe for the purpose of exhaust gas purification. In the case where the EGR pipe is equipped, a problem that has not been considered and that originates from the altitude is added to the problem of the torque step. This problem will be explained below with reference to FIG. 8.
FIG. 8 shows an example of a throttle opening degree Dthr, fresh air quantity Qa, an EGR opening degree Degr, intake pressure Pim and torque Tr in the case where the combustion mode is switched to the rich combustion (RICH) in an interval from time t0 to time t1 during the lean combustion (LEAN). In FIG. 8, H indicates the transition at the high altitude and L indicates the transition at the low altitude. The throttle opening degree Dthr means an opening degree of an intake throttle. The EGR opening degree Degr means an opening degree of an EGR valve.
As shown in FIG. 8, in the rich combustion, the intake throttle is adjusted to a predetermined opening degree smaller than in the lean combustion. Feedback control may be performed to achieve the predetermined opening degree of the intake throttle during the rich combustion. The intake air quantity decreases due to the decrease of the opening degree Dthr of the intake throttle. During the rich combustion, the EGR opening degree Degr is also set to a predetermined opening degree.
Under the above situation, the value of the intake pressure Pim during the rich combustion becomes smaller at the high altitude than at the low altitude because of the decrease of the outside air pressure. FIG. 8 shows the situation where the intake pressure Pim is 80 kPa at the high altitude whereas the intake pressure Pim is 100 kPa at the low altitude. The intake pressure Pim may be a pressure value in an intake manifold. The high altitude may be an altitude that is the highest in a range where running of the vehicle is normally expected and that necessitates catalyst control (NOx reduction) of the LNT. For example, the high altitude may be set as the altitude of 1800 m. 100 kPa and 80 kPa in FIG. 8 are approximate values.
At the high altitude, the sum total value of the fresh air quantity Qa suctioned into the cylinder and the EGR gas quantity Qegr decreases with the reduction in the intake pressure Pim. An example of such the situation is shown in FIG. 9. Pim in FIG. 9 indicates the pressure value in the intake manifold. In the sum total value, the fresh air quantity Qa is decided by the opening degree Dthr of the intake throttle and is the same quantity at both of the high altitude and the low altitude. Therefore, at the high altitude, the EGR gas quantity Qegr decreases with the reduction in the intake pressure Pim as shown in FIG. 9.
It can be assumed that the oxygen concentration is substantially zero in the EGR gas. Therefore, if the EGR gas quantity Qegr decreases, the oxygen concentration in the cylinder increases. Because of the increase of the oxygen concentration, the value of the generated torque Tr increases. Therefore, as shown in FIG. 9, the torque value Tr during the rich combustion in the case of the high altitude is larger than the torque value Tr during the rich combustion at the low altitude. Accordingly, as shown in FIG. 8, even when the torque step between the rich combustion and the lean combustion is avoided at the low altitude, the torque step occurs at the high altitude. Avoidance of such the torque step originating from the problem concerning the altitude has not been taken into consideration in the conventional technologies.